Struggling with measuring a screw using thread wires.

[woodchucker]

I need to cut 7/8 - 14 threads. I have to single point both interior and exterior threads. I want to make it close to standard.


I went into machinery's hand book and built a spreadsheet to do the calc so I can use it going forward.

Anyway, I have no way of testing the 7/8-14 so I though I would run the calc on 3/8-16 since the same wire can be used.
My measurement across the 3 wires is 0.386 That's out of range of the calc by both Pitch Diam and Major Diam.
see attached image, and spreadsheet... what am I doing wrong..

 

[John O]

Did you measure the wires?
0.0361 wires will come close to 0.386

 

[woodchucker]

yea they are on the money. since I used music wire, I had to measure, to make sure..
I always thought I had to use the exact size in the wire sets.. which I don't have.
Looking up MH I saw I can vary it.
So am I just dealing with a bolt that's not in spec but is close, or should I be in that range?
I just need to know I am doing the calc right to do the 7/8... the 3/8-16 was just a test.

 

[francist]

I used the online calculator I usually use but changed the wire diameter to yours. Here’s what it gave me for 3/8”-16 using 0.041 wires…

 

[woodchucker]

I used the online calculator I usually use but changed the wire diameter to yours. Here’s what it gave me for 3/8”-16 using 0.041 wires…

View attachment 381291

I must be missing something, I don't see what the measurement range between the wires should be.

edit: my bad, over wires... thanks.
so hmmm. The bolt that I have is under spec.
Thanks for confirming my numbers, although, they are different than mine.

 

[T Bredehoft]

In addtion to getting the right wires, the problem of holding them can be solved by sticking the three wires in a small block of Styrofoam. I learned this trick after retiring, wished I'd found it 20 years sooner.

 

[woodchucker]

In addtion to getting the right wires, the problem of holding them can be solved by sticking the three wires in a small block of Styrofoam. I learned this trick after retiring, wished I'd found it 20 years sooner.

already there with that.
I couldn't imagine juggling them.
thanks

 

[SLK001]

Here's my go-to site for calculating threads.

 

[macardoso]

Here's my go-to site for calculating threads.

I'll second this, use it a lot.

Ideally the wires are selected to fall right on the pitch diameter, although you'd need an infinite number of wires to make this happen. Realistically we settle for using wires that are relatively close to the ones we would like to use. This website recommends the best wires for a given thread size as well as nearby common sizes. You may also enter your own non-standard wire diameter, although you need to keep them between the stated min/max in the wire size drop down.

 

[woodchucker]

I'll second this, use it a lot.

Ideally the wires are selected to fall right on the pitch diameter, although you'd need an infinite number of wires to make this happen. Realistically we settle for using wires that are relatively close to the ones we would like to use. This website recommends the best wires for a given thread size as well as nearby common sizes. You may also enter your own non-standard wire diameter, although you need to keep them between the stated min/max in the wire size drop down.

yep, same with machinery's handbook calc. you pick wires between the min-max. they do show the optimum.
problem is I don't sync up with that website's calc. not far off, and the bolt I chose, didn't meet spec either...